FIELD OF THE INVENTION
The invention relates to a method and to a device for converting a pollutant in an exhaust gas in a catalytic converter using a reagent. The reagent is metered as a function of operational parameters of the exhaust gas and of a temperature of the catalytic converter and is added to the exhaust gas to form a mixture, and the mixture is fed through the catalytic converter. The device includes a reagent addition device for metering and adding a reagent to form a mixture with the exhaust gas which is then fed to the catalytic converter. The device also includes sensor elements for measuring operational parameters of the exhaust gas and a control unit for controlling the reagent metering as a function of the operational parameters and a temperature of the catalytic converter.
Such a method and such a device are disclosed by German Published, Non-Prosecuted Patent Application DE 43 15 278 Al, corresponding to U.S. Pat. No. 5,628,186, and German Patent DE 33 37 793 C2. According to each of these documents, which have a corresponding method or a corresponding device, a temperature measurement takes place in the catalytic converter. Operational parameters of the exhaust gas are additionally determined, in particular temperature, mass throughput, chemical composition and the like, and a reagent addition respectively required is controlled as a function of these parameters.
It is known to break down the nitrogen oxides resulting from fuel combustion in an internal combustion engine, after the addition of a reagent in the form of a reducing agent to the exhaust-gas flow which enters a downstream catalytic converter. Thus, nitrogen oxides are broken down in a reduction catalytic converter to nitrogen and water vapor by adding the reducing agent ammonia. The amount of reagent to be added must be matched accurately to the catalytic activity of the catalyst in order to avoid excess or deficit reagent.
For matching the amount of reducing agent to be added in the heat-up phase of a catalytic converter, German Patent DE 43 10 961 C1 discloses the use of a plurality of sensors for measuring temperatures inside of the catalytic converter, and, the provision of a matched addition of reducing agent taking into account the temperatures measured inside the catalytic converter, adapted to the catalytic activity of the sections of the catalytic converter upstream, between and downstream of the sensors. As a result, for example, a partial activation of the catalyst, already occurring in the heat-up phase, can be identified, and a matched amount of reducing agent can thereby be added even in the heat-up phase.
Furthermore it is known from Patent Abstracts of Japan No. JP 60 197225 to determine the temperature of the catalytic converter from the exhaust gas temperature at the inlet of the catalytic converter by the use of catalyst temperature simulators and to take it into consideration when determining the added amount of reducing agents with reference to the storage capacity of the catalytic converter.